- P. impressus, sculpture little accentuated, post. piliferic pits rudimentary.
- P. planus, sculpture more accentuated, post. piliferic pits lacking.
- P. tenuissimus, very small.
In my specimen, the two anterior rows of movable plates lack the marginal piliferous pits, on the next two rows they are rudimentary (which is also true of the lateral plates even further back), while on the bulk of the movable plates and on those of the pelvic buckler there are two, three or four good-sized piliferous pits on the rear. I can therefore recognize but two species, P. impressus and P. tenuissimus.
Prozaedius impressus Ameghino
- P. impressus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 508.
- P. planus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 509.
Our specimen was found on the Chico del Chubut River, west of Puerto Visser, and preserves over two hundred plates, and eight caudal vertebrae. The anterior rows of plates of the carapace consist of thin plates overlapping about a fourth their length. Just behind the overlap, there is, on each, a group of small punctations, and the exposed part of the surface is divided by two shallow furrows, making three more or less equal ridges which die out toward the rear, leaving the posterior part of the plate plain. These most anterior plates are bent to one side and have no piliferous pits on the rear margin. The plates of the third and fourth rows are not bent, and have the sculpture more distinct, the extreme lateral plates having no piliferous pits, the median lateral plates with rudimentary piliferous pits, and the dorsal ones with well marked posterior pits. In each succeeding row toward the rear, the plates are more distinctly ornamented and have larger posterior marginal pits. I have no marginal plates.
Fig. 132. Portion of carapace—natural size;
unshaded plates are from cast; a and
b plates from pelvic buckler.
The plates of the pelvic buckler do not overlap, are shorter, have a very distinct figure, and, in addition to the longitudinal furrows, have a couple of radial furrows on either side, which divide the plate into four or five areas ([see fig. 132 a and b]).
The caudal vertebrae are short and thick, indicating a short tail. I found no plates which would indicate a caudal shield, which coincides with the experience among the Santa Cruz specimens. The figures are to scale and give most of the measurements.
There are ten rows of movable plates, probably two to three rows lacking.